[POWERPC] XilinxFB: Replace calls to printk with dev_dbg, dev_err, etc.

The dev_dbg, dev_err, etc functions provide more context that plain
vanilla printk which is useful for debugging.  Where appropriate,
change printk calls to the appropriate dev_*() call.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andrei Konovalov <akonovalov@ru.mvista.com>